The invention relates to a method for determination of the content of at least one component of a sample by means of a nuclear magnetic resonance pulse spectrometer, with the magnetization of the sample being influenced by a sequence of radio-frequency pulses such that the signal amplitudes to be observed can be determined.
A method of the type mentioned above, which is also referred to as time domain nuclear magnetic resonance measurement (time domain NMR, TD-NMR), is generally known and has been used for many years for the determination of the content of specific components in foods, for example for determination of the fat content in food products.
The time domain nuclear magnetic resonance measurement methods which have been used so far for determination of the fat or water content are restricted to samples which have a relatively low free water content of less than about 12%. This leads to the determination of the fat content of a food using a low-resolution nuclear magnetic resonance (NMR) pulse spectrometer in products with a high water content being made more difficult owing to the superimposition of the water and fat signals. Low-resolution nuclear magnetic resonance (NMR) pulse spectrometers are commercially available with a magnetic field strength of a maximum of about 1.5 Teslas and with a mean homogeneity of about 10−5 over the sample volume, and thus with a proton resonant frequency below about 60 MHz, for example the “minispec” from the Bruker Company. In other words, the determination of the fat content of samples with a relatively high water content is easily possible only by using high-resolution NMR spectrometers which, however, are generally much too expensive for this application, for example for foods chemistry.
In traditional time domain nuclear magnetic resonance (NMR) methods, only a single signal amplitude is obtained at a specific time or amplitude ratio and is compared with the results of a reference method for determination of the content of the component in the sample. For example, the oil content in seeds is determined by measurement of the spin echo amplitude for a specific echo time.
One possible way for also obtaining acceptable results for samples with a water content of more than about 12% by means of low-resolution nuclear magnetic resonance pulse spectrometers is to pre-dry the sample before the NMR measurement, for example in a drying oven, microwave drier or infrared drier, or by means of chemical drying, for example as described in DE 41 33 643 C1, in order to get rid of or at least to reduce the disturbing water component.
The disadvantage in this case is that the method for pre-drying requires a further process step which, depending on the method, is more or less labor-intensive and, furthermore, requires drying equipment where, in addition, the measurement process requires two much time owing to the pre-drying step, which is unacceptable for a large number of measurements which are required in the foods industry.
WO 99/54751 A1 and WO 01/92908 A1 describe NMR pulsed methods in which the different self-diffusion coefficient is also made use of, in addition to the different relaxation times T2 of water and fat, in order to obtain a statement about the pure fat content. The entire content of these two documents is included in the disclosure content of the present application.
The basis of the methods which are known from the previously cited two documents for determination of the content of at least one component in a sample is the so-called PFGSE method (Pulsed Field Gradient Spin Echo), which is known per se from textbooks, for example from P. T. Callaghan: Principles of Nuclear Magnetic Resonance Microscopy, Oxford Science Publications, Clarendon Press, Oxford 1991 and in particular pages 162-169, 330-367, 371-417 and 478-482 there).
However, even the methods mentioned above which use the different self-diffusion coefficients in order to determine the content of at least one component in a sample have weaknesses.
Specifically, the gradient method that is described in these documents requires precise gradient control in order to produce the corresponding gradient pulses which, furthermore, themselves induce eddy currents in the apparatus, and these can adversely affect the measurement. Furthermore, the apparatuses that are currently commercially available are limited by the space required for the gradient coils, thus affecting the sample dimensions.
The known methods are based on the idea that only a single measurement is taken of a single relaxation time, that is to say the longitudinal relaxation time T1 or the transverse relaxation time T2.
Furthermore, the article by F. C. Tinsley et al., “Evaluation of a Quantitative Magnetic Resonance Method for Mouse Whole Body Composition Analysis” in Obesity Research, Vol. 12, No. 1, January 2004, describes a method in which the fat and water content of live mice is determined by means of an NMR pulse sequence, thus allowing the T1 and T2 influences to be measured. In this case, the measurement is calibrated using synthetic samples.